(1) Field of the Invention
This invention is related to MEMS devices, and more particularly, to methods of forming a MEMS scanning mirror with tunable resonance frequency.
(2) Description of the Related Art
Micro-electro-mechanical systems (MEMS), the smallest functional machines that can be manufactured currently, are made up of components ranging from a few micrometers to several millimeters in size. MEMS, a rapidly growing semiconductor field, have many important practical and potential commercial applications. MEMS scanning mirrors have been found in many applications such as barcodes, laser printers, projection displays, and rear projection TV's, for example. A MEMS scanning mirror is formed by the mirror itself, anchors, and a spring coupling the mirror and the anchors. Such a MEMS scanning mirror could be driven by either magnetic or electrostatic force. The MEMS scanning mirror device of the present invention is based on an electrostatic comb actuator.
A MEMS scanning mirror is usually driven at its resonance frequency under alternating current (AC) to achieve a high scanning angle. For different applications and products, the required resonance frequency might be different. The resonance frequency of a MEMS scanning mirror depends on the mirror's mass, its dimension, and materials properties such as the elastic modulus. The resonance frequency can be pre-determined through mask design and it can be adjusted by mirror mass change during mirror fabrication.
Adjusting resonance frequency through mirror mass change has been proven efficient. U.S. Patent Application 2009/0080051 to Yoon et al discloses removing a portion of the backside of a moving plate to a predetermined depth to reduce the weight of the moving plate and increase resonance frequency. However, the etching is performed without patterning.
U.S. Pat. No. 7,187,488 and U.S. Patent Application 2007/0117051, both to Fu, disclose trimming a sacrificial portion of a mirror by laser beam or ion beam. Since there is no etch selectivity with those methods, etch uniformity and frequency control can be very poor.
U.S. Pat. No. 7,217,587 (Fu) notes removing a mass by laser trimming, but using different steps from the invention.